The acridine double intercalators inhibit the synthesis of nucleolar RNA in HeLa cells. The study of a homologous series of diacridines, interconnected through a 9-NH2 group by paraffinic chains of varying lengths, shows that they have different specificities of action - some inhibit rRNA processing, others enhance the degradation of rRNA while others inhibit rRNA methylation. Studies on the uptake of these compounds by HeLa cells also show different specificities.